J. M. Segenhout

Two-phase endolymphatic hydrops: A new dynamic guinea pig model

The classical guinea pig model for Menieres disease, in which endolymphatic hydrops was achieved by destruction of the endolymphatic sac and obliteration of the endolymphatic duct, is a non-physiological profound model with shortcomings in relation to Menieres disease as seen in patients. We developed a more subtle animal model; the two-phase endolymphatic hydrops. This model is based on a combination of chronic endolymphatic sac dysfunction, induced by slight destruction of the most distal part of the endolymphatic sac, and acute stress-induced endolymph production by stimulation of the Na/K-ATPase in the stria vascularis with aldosterone. Light microscopy of the fluid compartments of four groups of cochleas was used to examine them for the presence of endolymphatic hydrops: i) Normal (control) cochleas showed no hydrops; ii) some of the non-operated (no destruction) aldosterone-treated cochleas showed small degrees of hydrops mainly present in the basal turns; iii) mild dissection of the endolymphatic sac without administration of aldosterone produced a hydrops which was mainly present in the cochlear apex; iv) combination of chronic endolymphatic sac dysfunction and acute attacks of endolymph production by aldosterone administration revealed the most severe degrees of hydrops in all cochlear windings, damage to cochlear structures, and cellular disturbances of the epithelial lining of the endolymphatic sac. This new model may represent a more physiologic and dynamic approach to Menieres disease and may explain the etiology of many symptoms in patients such as the fluctuant nature and the types of sensoneuronal hearing losses.

WIENER KERNEL ANALYSIS OF INNER-EAR FUNCTION IN THE AMERICAN BULLFROG

The response of 17 primary auditory nerve fibers in the American bullfrog (Rana catesbeiana) to acoustic noise stimulation of the tympanic membrane was recorded. For each fiber, the first- and second-order Wiener kernels, k1 (tau 1) and k2 (tau 1, tau 2), were computed by cross correlation of the stimulus and the response. The kernels revealed amplitude and phase characteristics of auditory filters of both phase-locking and non-phase-locking fibers. Wiener kernels of high- and midfrequency fibers (best frequency, BF > 500 Hz), implied a simple sandwich model, consisting of a cascade of a linear bandpass filter, a static nonlinearity, a linear low-pass filter, and a spike generator. The bandpass filter was at least of order 7, and had a linear phase response, for both the high- and the midfrequency fibers. Averaged across fibers, filter order 2, and cutoff frequency 451 Hz for the second filter in the model was observed. The responses of low-frequency fibers (BF < 500 Hz) could not be fit with the sandwich model, because the Fourier transform K2 (f1,f2) of the second-order Wiener kernel showed significant components at off-diagonal frequencies f1 not equal to +/- f2. The presence of these off-diagonal components shows that, in addition to the phase and gain characteristics of auditory filters, the Wiener kernel analysis reveals nonlinear two-tone interactions.

Dynamics of Inner Ear Pressure Change Caused by Intracranial Pressure Manipulation in the Guinea Pig

Previous studies have shown that pressure changes in the cerebrospinal fluid compartment are transmitted to the inner ear. The main route for pressure transfer is the cochlear aqueduct, about which little is known with regard to its dynamic properties. In the present study, sudden intracranial pressure changes (square waves and short pulses) were created in guinea pigs by means of an electronically controlled infusion system. Simultaneously with pressure manipulation, hydrostatic pressure was monitored in both the peridural space and the perilymphatic compartment of the inner ear. The onset of an inner ear pressure change following manipulation of intracranial pressure was immediate. Inner ear pressure increased or decreased without a measurable time lag, and equalized within a few seconds. During square wave intracranial pressure manipulation, inner ear pressure equalized somewhat more slowly after pressure increase than after pressure decrease. To a first approximation, the pressure equalization curves for the inner ear could be fitted with a single exponential function, rising or falling with a time constant in the range 1-3 s, and the system can be described as a low-pass filter composed of a constant compliance and a constant flow resistance. Detailed analysis, however, showed small deviations from a purely exponential recovery process. With a more complicated (non-linear) model, almost perfect fits to the inner ear pressure equalization curves could be obtained. This non-linearity may be a consequence of the dependence of the compliance and/or flow resistance on pressure.

Morphology and function of Bast’s valve: additional insight in its functioning using 3D-reconstruction

The utriculo-endolymphatic valve was discovered by Bast in 1928. The function of Bast’s valve is still unclear. By means of orthogonal-plane fluorescence optical sectioning (OPFOS) microscopy 3D-reconstructions of the valve and its surrounding region are depicted. The shape of the duct at the utricular side is that of a flattened funnel. In the direction of the endolymphatic duct and sac this funnel runs into a very narrow duct. The valve itself has a rigid ‘arch-like’ configuration. The opposing thin, one cell-layer thick, utricular membrane is highly compliant. We propose that opening and closure of the valve occurs through movement of the flexible base/utricular membrane away from and toward the relatively rigid valve lip.

Three-dimensional reconstruction of the guinea pig inner ear, comparison of OPFOS and light microscopy, applications of 3D reconstruction.

Three‐dimensional (3D) reconstruction of anatomical structures can give additional insight into the morphology and function of these structures. We compare 3D reconstructions of the guinea pig inner ear, using light microscopy and orthogonal plane fluorescence optical sectioning microscopy. Applications of 3D reconstruction of the inner ear are further explored. For each method two bullas were prepared for 3D reconstruction. Both methods are explained. In general, the 3D reconstructions using orthogonal plane fluorescence optical sectioning microscopy are superior to light microscopy. The exact spiral shape of the cochlea could be reconstructed using orthogonal plane fluorescence optical sectioning microscopy and the length of the basilar membrane measured. When a resolution of 20 μm is sufficient, orthogonal plane fluorescence optical sectioning microscopy is a superior technique for 3D reconstruction of inner ear structures in animals.

The relationship of the round window membrane to the cochlear aqueduct shown in three-dimensional imaging

The round window membrane and cochlear aqueduct complex in the guinea pig are reconstructed with 3D-imaging, using orthogonal plane fluorescence optical sectioning (OPFOS). The 3D-images show that the periotic duct and the aqueduct are connected to a pouch-like extension of the round window. The function of this may be regulation of aqueduct flow resistance under the influence of a pressure difference between inner ear fluid and middle ear.

Mechanics of the exceptional anuran ear

The anuran ear is frequently used for studying fundamental properties of vertebrate auditory systems. This is due to its unique anatomical features, most prominently the lack of a basilar membrane and the presence of two dedicated acoustic end organs, the basilar papilla and the amphibian papilla. Our current anatomical and functional knowledge implies that three distinct regions can be identified within these two organs. The basilar papilla functions as a single auditory filter. The low-frequency portion of the amphibian papilla is an electrically tuned, tonotopically organized auditory end organ. The high-frequency portion of the amphibian papilla is mechanically tuned and tonotopically organized, and it emits spontaneous otoacoustic emissions. This high-frequency portion of the amphibian papilla shows a remarkable, functional resemblance to the mammalian cochlea.

Sensory cell damage in two-phase endolymphatic hydrops: a morphologic evaluation of a new experimental model by low-voltage scanning techniques.

Hypothesis The aim of this study was to create a more dynamic animal model of Ménières disease combining multiple causes, such as the role of endocrine factors and endolymphatic sac dysfunction, that may mimic the fluctuant characteristics of Ménières disease. Background Endolymphatic hydrops remains to be considered a pathologic substrate in the etiology of Ménières disease. The classic guinea pig model of inducing hydrops by total destruction of the endolymphatic sac is a nonphysiologic rigid model of Ménières disease. Methods The authors developed the two-phase endolymphatic hydrops model by inducing hydrops by mild chronic endolymphatic sac dysfunction, in combination with increased endolymph production by aldosterone. Sensory cell damage was evaluated by low-voltage field emission scanning microscopy. Results This study describes a wide spectrum of morphologic effects of the outer hair cells in radial gradients, in which most effects were observed in the third to second row of outer hair cells, and longitudinal gradients in which the most severe effects were observed in the apical turns. Most affected were the ears that underwent distal endolymphatic sac dissection followed by the administration of aldosterone. Damaging effects proceeded from degeneration and absence of short stereocilia of outer hair cells and even some inner hair cells in the apical turns, to stereociliary disarrangement and atrophy, followed by degeneration and absence of outer hair cells, which were replaced by supporting cells. Conclusion The two-phase endolymphatic hydrops model seems to represent a functional model that may mimic the fluctuant characteristics of Ménières disease and emphasizes the influence of multiple and coexisting hydrops-inducing influences.

Cochlear aqueduct flow resistance depends on round window membrane position in guinea pigs

The resistance for fluid flow of the cochlear aqueduct was measured in guinea pigs for different positions of the round window membrane. These different positions were obtained by applying different constant pressures to the middle ear cavity. Fluid flow through the aqueduct was induced by small pressure steps superimposed on these constant pressures. It was found that the resistance for fluid flow through the aqueduct depended on the round window position but not on flow direction. The results can be explained by special fibrous structures that connect the round window with the entrance of the aqueduct. It was also found that the equilibrium inner ear pressure depends on middle ear pressure, indicating that the aqueduct does not connect the inner ear with a cavity with constant pressure.

Monitoring inner ear pressure changes in normal guinea pigs induced by the Meniett (R) 20

The inner ear fluid pressure of guinea pigs was measured during a series of complex oscillating middle ear pressure changes induced by the Meniett®20 (Pascal Medical, Sweden), a possible therapeutic pressure generator to be used by patients with Ménières disease. Middle ear pressure changes were transferred instantly to the inner ear, although inner ear pressure declined while middle ear pressure stayed relatively stable. An average undershoot of -1.0 cm H2O with respect to the steady-state pressure was seen after application of a pressure pulse, which was released in a few seconds. The results did not fully comply with a simple linear model in which a constant flow resistance between the inner ear and cerebrospinal space was assumed.